1. Field of the Invention
This invention pertains to an improved composition for the storage of heat as, for example, in the solar heating of buildings. More specifically the invention pertains to an improvement in heat storage systems which employ the heat of fusion of sodium sulfate decahydrate, whereby the capacity of the system to store heat is improved during a large number of freeze-thaw (thermal) cycles.
2. Description of the Prior Art
Sodium sulfate decahydrate, hereinafter called Glauber's salt has long been a candidate for the storage of sensible heat, particularly in the solar heating of houses. In addition to its cost and ready availability, the physical and chemical properties of the salt are especially attractive in this use: it has a convenient melting temperature of about 32.degree. C. and a large heat-of-fusion of 50-60 cal/g. The melting temperature of Glauber's salt can be depressed to as low as 4.degree. C. by the addition of suitable salts. Such low melting point mixtures have been suggested for the storage of "cold" to assist in air conditioning.
However, Glauber's salt melts incongruently, spontaneously nucleates the undesired heptahydrate, and severely undercools. The addition of small amounts of borax (Na.sub.2 B.sub.4 O.sub.7.10 H.sub.2 O), which is a nucleator for Glauber's salt, as disclosed by Telkes in U.S. Pat. No. 2,677,664, has solved the latter two problems.
However, the problems associated with incongruent melting are subtle and have not heretofore been completely solved. On melting, Glauber's salt forms Na.sub.2 SO.sub.4 and water. The Na.sub.2 SO.sub.4 does not completely dissolve in the released water of crystallization. Consequently about 15% of the solid Glauber's salt precipitates as Na.sub.2 SO.sub.4 which, being more dense than the solution, settles to the bottom of the container. In the absence of deliberate homogenization, lowering the temperature below the melting point of Glauber's salt results in the formation of a metastable condition wherein the undissolved Na.sub.2 SO.sub.4 coexists with newly formed Glauber's salt and excess solution.
For the purpose of storing thermal energy, this metastable condition is undesirable since the system's energy storage capacity is determined by the amount of Glauber's salt formed. Thus, when a melt of pure Glauber's salt is cooled below its melting point, not all of the potentially available salt is reformed in a practical time. Reestablishment of the single phase of Glauber's salt is retarded as the newly formed Glauber's salt crystallizes on top of the layer of Na.sub.2 SO.sub.4 thereby impeding the desired recombination of Na.sub.2 SO.sub.4 and the water of the solution.
Telkes, in U.S. Pat. No. 3,986,969, suggested the addition of attapulgite clay, which forms a thixotropic solution, to suspend the Na.sub.2 SO.sub.4 in the melt. Other thickeners have also been proposed. For example, Kent et al in European Patent Application No. 0 011 411 proposed the use of covalently cross-linked sodium polycarboxylates, among other materials. Such means, although useful, have provided only partial solutions to the problem. Solution of the problem is particularly difficult because heat storage compositions, in practical use, must undergo many hundreds of freeze-thaw cycles without excessive degradation in heat storage capacity.